Hydrogeology of the London Basin

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'Advances in understanding the hydrogeology of the London Basin', poster presented at the London Basin Forum meeting, 14 October 2010.

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Hydrogeology of the London Basin

  1. 1. Advances in understanding the hydrogeology of the London Basin Steve Buss1 and Travis Kelly2 With grateful acknowledgements to the rest of the project team: Mike Jones, Rob Sage, Peter Isherwood, Vin Robinson, Rory Mortimore, Malcolm Anderson, Nigel Hoad, Jane Dottridge, Mike Streetly, Paul Daily, Victoria Price and Nancy Proudfoot Background The Chalk and Thanet Sands aquifer provides the main groundwater resource in the London area and supports significant abstraction for a variety of uses including public water supply. Beneath London the aquifer is confined by clayey strata of the Lambeth Group and London Clay. Outcrop geology of the study area is mapped below. Stratigraphic control of aquifer properties It has been recognised that Chalk aquifer properties can be correlated to the new stratigraphic classification. The uppermost strata beneath London, the Newhaven and Seaford Chalk formations, have well-developed intersecting fracture sets and therefore readily permit groundwater flow. Beneath these the Lewes Nodular Chalk, while also relatively well-fractured, contains several tabular flint and marl horizons which restrict vertical groundwater flow. Deeper formations are less well fractured because they are more clay-rich and plastic (although aquifer properties can be developed at outcrop). Re-analysis of existing data: hydraulic properties chalk core from a borehole at Faircross, near Reading (Bloomfield, 1997), and numerous down-hole geophysical logs from the Environment Agency, show physical evidence of this. Clear demarcations between regions of similar aquifer properties are seen at formation-level stratigraphic boundaries. In some of the down-hole logs, properties can be seen to vary at bed-level. Burial depth control of aquifer properties The figure (right) compares the distribution of transmissivity from Downing et al. (1972) with the current depth of the top of the Chalk aquifer. There is excellent correspondence between areas where the depth exceeds 80 m and the areas of identified low transmissivity. Topography associated with river valleys is reflected in the depth of burial: notably the River Lee and River Wandle both lie above areas in the aquifer where the Chalk would be buried deeper than 80 m if it were not for the overlying river valleys. Some areas (e.g., beneath the River Roding) have the top of the Chalk above 80 m below ground level but apparently low transmissivity. However, since there is no link from these areas to outcrop, a flux of groundwater to cause solution enhancement cannot be provided. Similarly, there is not a high transmissivity channel beneath the River Hogsmill because there is nowhere for the fresh groundwater to discharge. Structural controls on groundwater flow Faulting often alters the hydraulic conductivity of Chalk: both to increase and decrease permeability. Along the length of faults, permeability can be enhanced by fracturing (then can be further enhanced by dissolution). However, after displacement the permeability can be reduced by mineralisation. Field evidence and the results from existing calibrated models demonstrate the importance of both increased and reduced permeability of faults in the basin. A new interpretation of the fault pattern in Central London was provided for the project by the BGS. Groundwater levels either side of each major fault were reviewed to assess whether consistently steep hydraulic gradients developed across the faults. Most of the major faults (shown in the figure right) demonstrated discontinuity of groundwater levels. After this, all groundwater level data for 2000 and 2007 were contoured based on this interpretation. Groundwater levels appear to be particularly discontinuous in the south of the area, where there is most faulting (figure upper right). Using high resolution stratigraphic data from the BGS geological model, and from a version of the existing London Basin Groundwater Model, we were able to compare these with the interpreted groundwater level surface. The figure right shows the formation that the water table/piezometric surface was in, in 2007. There are considerable areas beneath the London Clay where the Chalk and Thanet Sands aquifer is unconfined. T = 500 m/day Tx:Ty = 5 TF = 5000 m/day TF = 50 000 m/day TF = 50 000 m/day TF = 50 m/day TF = 0 m/day TF = 0 m/day High or low permeability faults? Despite the evidence that there are steep hydraulic gradients perpendicular to some fault zones, there appears to be no direct evidence that these zones are less permeable than the surrounding aquifer. There are, however, several lines of field evidence that suggest some fault zones may be more permeable than the aquifer. Simple numerical modelling of a pumping test in an otherwise homogeneous aquifer shows how: 1. Narrow zones of high permeability cause a change in the aspect ratio of the cone of depression: its long axis develops parallel to the fault zones. This is similar to what would be observed in an anisotropic aquifer. 2. Drawdown does not propagate across the high permeability zones, resulting in steep hydraulic gradients perpendicular to them. 3. While low permeability zones also lead to steep hydraulic gradients, the long axis of the cone of depression forms perpendicular to the fault zones. We learn from this that quantification of these effects in the field, and indeed the discrimination of narrow high permeability zones from narrow low permeability zones from regional anisotropy, depends on using several strategically-placed observation boreholes, and having an open mind when it comes to pumping test analysis. Sources of groundwater The Trafalgar Square hydrograph clearly shows that historically groundwater demand exceeded supply and that the abstraction considerably depleted aquifer storage. Having available the results of adjacent groundwater models, particularly: the South West Chilterns, Vale of St Albans, Essex, and Swanscombe models, we have been able to determine inflows to the aquifer from other outcrop areas. The plot below shows groundwater inputs vs. outputs for the confined London Basin since 1965. Abstraction from the confined aquifer is currently approximately balanced by inflows from the Chilterns and North Downs (which is an indication of the success of the GARDIT programme). Compare the total inflows below with the maximum historical abstraction of almost 500 Ml/day. ABOVE (after Bloomfield, 1997): Even deeply confined, and without solution enhancement, the Seaford Chalk exhibits typically high porosity; then there is a gradual decrease within the Lewes Nodular Chalk to the Chalk Rock at its base. Below that the porosity is variable. This transition (at about 155 m depth) is a transition between diagenesis due to mechanical compaction above, and pressure solution compaction below. The overall decline in porosity is matched by a corresponding, and more regular, decrease in the gas permeability of intact chalk. LEFT: Most boreholes penetrate about 50-60m of the Chalk because hydrogeologists understood that the top 60 m was the zone of groundwater flow and drilling deeper would be into progressively less productive Chalk. Several boreholes show a zone of widening around the Cuckmere Beds, which are a particularly weak stratum in the middle of the Seaford Chalk Formation. Others show development at the sub-Palaeogene unconformity, possibly due to the effects of acid groundwater from the Thanet Sands. This poster presents some of the findings of an Environment Agency-funded study into the hydrogeology of the London Basin aquifer. This conceptual model study provides a robust, quantitative foundation for a forthcoming numerical modelling project. The numerical model will be used by the Environment Agency to regulate the diverse abstraction pressures on the aquifer. Historical abstraction caused the development of a regional cone of depression beneath Central London, in the centre of which the Chalk aquifer developed an unsaturated zone. Since 1960 there has been a rise in groundwater levels over most of the Central London area as a response of the reduction in pumping. This recovery would potentially cause issues with structural integrity of infrastructure: this has led to a strategy of increasing abstraction in the area (GARDIT: General Aquifer Research Development and Investigation Team). Aquifer storage and recharge (ASR) schemes have been developed in the areas where historical abstraction caused the development of an unsaturated zone in the Thanet Sands. -400 -300 -200 -100 0 100 200 300 400 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Flows(Ml/day) River Thames Storage Chilterns North Downs Net Abstraction FlowintoaquiferFlowoutofaquifer 2007 (all elevations in m AOD) More details on the project and its outcomes can be obtained from the authors: 1 Dr Steve Buss, ESI Ltd., 160 Abbey Foregate, Shrewsbury SY2 5HH (stevebuss@esinternational.com) 2 Travis Kelly, Environment Agency, Red Kite House, Howbery Park, Wallingford OX10 8BD 2007

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